1. Field of the Invention
The present invention relates to a method of a surface treatment, and more particularly, to a method of a surface treatment on a fluorinated silicate glass (FSG) film.
2. Background of the Invention
With the decreasing size of semiconductor devices and an increase in integrated circuits (IC) density, an intermetal dielectric layer must have certain characteristics such as a low dielectric constant, good gap-filling properties, good mechanical stability and a low water absorption capability, etc. In addition, a reduction in the dielectric constant of an intermetal dielectric layer can enhance the speed of transmitting signals due to a related reduction in a parasitic capacitance between metal wires. Silicon dioxide is a typical dielectric material but its dielectric constant (kxcx9c4) is too high for present semiconductor fabrication. As a result, a material with lower dielectric constant such as FSG (kxcx9c3.5) is required to replace silicon dioxide as an intermetal dielectric layer. Since fluorine is an electronegative atom, it can effectively reduce polarizability in a SiOF network of silicon dioxide and therefore reduce the dielectric constant of silicon dioxide. Additionally, fluorine is a strong etching species and it will etch the deposited FSG film to produce a deposition/etching effect. Moreover, a void-free FSG film is obtained in semiconductor fabrication with a thinner line width.
Please refer to FIG. 1 to FIG. 3, FIG. 1 to FIG. 3 are schematic diagrams of a prior method of forming a patterned photoresist on a fluorinated silicate glass (FSG) film. As shown in FIG. 1, a semiconductor wafer 10 comprises a substrate 12, a fluorine silicate glass layer 14 deposited on the substrate 12 and a photoresist layer 16 coated on the fluorine silicate glass layer 14. As shown in FIG. 2, exposing the photoresist layer 16 forms a pre-determined latent pattern 16a in the photoresist layer 16. As shown in FIG. 3, developing the photoresist layer 16, that is known by those skilled in the art, removes the pre-determined latent pattern 16a to expose corresponding portions of the fluorine silicate glass layer 14 underneath the pre-determined latent pattern 16.
Adding fluorine ions into silicon dioxide obtains a fluorine silicate glass film with a lower dielectric constant. However, fluorine ions in the fluorine silicate glass layer 14 are not stable meaning they might diffuse to the surface of the fluorine silicate glass layer 14. In addition, the fluorine silicate glass is so hydrophilic that it absorbs water molecules that react with the fluorine ions to form hydrofluoric acid (HF) on the surface of the fluorine silicate glass. Hydrofluoric acid reacts with the photoresist layer 16 and contaminates the photoresist, which leads to a formation of non-vertical sidewalls of exposed photoresist and moreover leads to collapse of exposed photoresist (as shown in FIG. 3). These effects cause imperfect transfer of the photoresist pattern to the semiconductor wafer. Additionally, hydrofluoric acid corrodes metal wires, which increases metal resistivity and causes reliability issues in integrated circuits.
It is therefore a primary objective of the present invention to provide a method of a surface treatment on a fluorine silicate glass film to solve the above mentioned problems.
The present invention provides a method of a surface treatment on a fluorine silicate glass film; the method involves depositing a fluorine silicate glass layer on a semiconductor wafer. In-situ removing fluorine ions from an upper surface of the fluorine silicate glass film to form a silicon oxide layer of a pre-determined thickness, and coating a photoresist layer on the silicon oxide layer. The photoresist layer is exposed to form a pre-determined latent pattern in the photoresist layer. The layer is developed and the pre-determined latent pattern of the photoresist is removed so as to expose corresponding portions of the silicon oxide layer underneath the latent pattern of the photoresist layer.
The present invention utilizes an oxygen-containing plasma to in-situ remove fluorine ions from an upper surface of the fluorine silicate glass film to form a silicon oxide layer of a thickness between 100 and 200 xc3x85. The method has the following advantages. (1) In-situ removing fluorine ions of the fluorine silicate glass film does not change the physical properties of the fluorine silicate glass film, such as thickness, refractive index and dielectric constant. (2) The thin silicon oxide layer, produced by the oxygen-containing plasma, prevents fluorine ions in the fluorine silicate glass from out-diffusion. (3) In-situ removing fluorine ions of the fluorine silicate glass film reduces the water absorption capability of the fluorine silicate glass. (4) Compared with depositing a silicon dioxide layer and a silicon oxynitride (SiON) layer to prevent fluorine ions in the fluorine silicate glass from out-diffusion in the prior art, the present invention has further advantages of reducing process steps and saving production cost.